Method of producing a sintered diamond compact

ABSTRACT

A sintered compact for use in a cutting tool and a method of producing the same are disclosed. The compact comprises 95 to 20 volume % of diamond finer than one micron in size and the balance binder also finer than one micron selected from the group of WC, (MoW)C, WC base cemented carbide and (MoW)C base cemented carbide. 
     The method comprises pulverizing a diamond powder by using cemented carbide balls and a pot having a cemented carbide lining, mixing the diamond powder with a powder abraded from the balls and pot to produce a powder mix finer than one micron containing 95 to 20 volume % of diamond, heat-treating the powder mix in vacuum so as to degas, and hot-pressing the powder mix under high pressure at high temperature within the stable range of diamond.

This is a division, of application Ser. No. 902,812 filed May 4, 1978 inour U.S. Pat. No. 4,171,973.

The present invention relates to a sintered compact for use in a cuttingtool and a method of producing the same.

A material which comprises a sintered compact containing more than 90volume % of diamond with a binder metal mainly composed of cobalt, and acemented carbide substrate having the sintered compact integrally bondedthereto is sold at the market for use in a cutting tool for cutting anonferrous alloy, synthetic resin, ceramics and the like. Although thematerial is expensive, it is highly estimated by some people for cuttingaluminium alloy containing relatively large amount of silicon, copperalloy having high hardness or the like.

The present inventors have examined various properties of this toolmaterial. When a cutting tool was formed by the material and wasactually used in cutting, it was assured that the material is much moreexcellent than a conventional cemented carbide tool in respect to wearresistance, and is more advantageous in toughness against shock ascompared to a tool formed from a single crystal natural diamond.

On the other hand of these advantages, after cutting, for example, anonferrous alloy, a machined surface thereof was examined to be foundthat the surface is rougher than the case in which a single crystalnatural diamond tool is used. Therefore, the material is not suitablefor machining a workpiece which should be finished specially smoothly.

Further, when a smaller workpiece such as watch element or a thinnerworkpiece was cut, there occured such a problem that the workpiece isdeformed or a dimension precision cannot be maintained due to the heavycutting resistance. As a result of the study, the following reasons wererevealed.

FIG. 1 is a microphotograph showing a cutting edge of a cutting toolformed by a marketed ultrafine WC (tungsten carbide)-Co cementedcarbide. FIG. 2 is a microphotograph showing a cutting edge of a cuttingtool formed from a marketed sintered diamond compact. As seen from thephotographs, the cutting edge of the tool from the marketed diamondcompact is not sharp and straight, but has numerous fine chippings. Thematerial was formed into a cutting tool by using a diamond grindingwheel. However, since the diamond wheel has a high grinding resistancewhen grinding the diamond compact, and soon clogs with swarf to becomedull, it is difficult to machine the material into a sharp cutting tool.

FIG. 3 is a microphotograph showing a commercially available diamondcompact which has such a structure that the diamond particles having aparticle size from 3 to 10 micron are in contact to one another. Whenthe diamond compact having the particle size of this range is ground bya diamond wheel, the diamond particles at a cutting edge are broken, anda sharp edge is not obtained. Particularly, if a cutting edge is to beof a shape such that the cutting edge angle formed by the rake face andflank has a positive rake angle less than 90°, it is impossible toobtain a satisfactory cutting edge from the marketed diamond compact.

One of the tool materials of marketed diamond compacts contains diamondparticles more than about 60 micron in size, as shown in FIG. 4. This ismainly used as a material for a wire drawing die.

The inventors have examined the properties as a wire drawing die inrespect to this sintered compact having coarser particle size. Severalexamples taught us that the wear resistance is considerably improvedwhen the die of this material is used in the work to which aconventional cemented carbide die is applied. However, the experimentsalso revealed several problems, one of which is that scratches remain onthe surface of a wire. FIGS. 5 and 6 show such examples. FIG. 5 showsthe surface condition of a copper wire with a diameter of 0.5 mm whichis produced by a die of a single crystal natural diamond. FIG. 6 showsthe surface condition of the same wire which is produced by a die of themarketed diamond compact. As apparent from the photographs, there aremany scratches on the surface of a wire which is drawn by the die of themarketed diamond compact. In order to investigate the cause, the innerperiphery of the die is examined after drawing a wire. As shown in FIG.7, a part of diamond particles is destroyed to be torn off. Thedeficient portion may bite a wire to be drawn, thus causing scratches.

The inventors have made various studies so as to develop a novel toolmaterial of sintered diamond compact which eliminates the defects in theconventional sintered diamond compact, and have obtained an idea thatthe defects might be obviated by using a diamond powder having a veryfine particle size. According to the idea, the inventors have made asintered diamond compact on trial.

For producing a sintered diamond, there is a process in which a diamondpowder mixed with a powder such as iron group element which can dissolvediamond is hot-pressed at high temperature under high pressure in thestable region of diamond, as disclosed, for example, in the officialgazette of Japanese Patent Publication No. 39-20483. The inventors havetried to make a compact by sintering a mix of diamond powder whichparticle size less than one micron and carbonyl nickel powder undersuper-pressure. A densely concentrated compact was obtained. However, asa result of examination of the structure of the compact, an abnormalgrowth of diamond crystals was observed all over the structure, andthere were number of particles having a particle size more than 500micron. The inventors have made experiments by varying the particle sizeof diamond powder as well as conditions of temperature and pressure uponsintering. As a result, it was found that when a particle size ofdiamond powder is more than 3 micron, the abnormal crystal growth willnot occur, and dense structure can be obtained, while if a particle sizeis less than one micron, the crystal growth always occurs under theconditions which allow the production of dense structure. That is,according to the method, a structure containing fine and uniformparticles of diamond cannot be obtained.

There is an another method which is disclosed in Japanese PatentPublication No. 52-12126. The method is thought to be a process ofproducing the commercially available diamond compact at present.According to the method, a diamond powder is put on a cemented carbidesubstrate in a cup, and is sintered at high temperature under highpressure to cause a liquid phase of Co-W-C eutectic mixture producedfrom the substrate to permeate into the diamond powder.

The inventors have tried to produce a compact by sintering undersuper-pressure a diamond powder of less than one micron which is put ona WC-6% Co cemented carbide disk in a cup. The interface between theobtained sintered compact and the cemented carbide substrate wasexamined to be found that there are many particles which abnormallygrows to several hundred micron.

Further, the sintering conditions were varies, but the abnormal crystalgrowth was always observed under the conditions in which a denselyconcentrated compact is obtained.

As a result of the above experiments, it was revealed that it isdifficult to produce a sintered diamond compact which has a uniformstructure consisting of diamond particles finer than one micron by theconventional methods known to the art.

It is therefore an object of the present invention to obviate the abovedefects, and to provide a sintered compact for a cutting tool havinguniform structure constructed by diamond particles finer than onemicron.

It is an another object of the present invention to provide a method ofproducing a sintered compact which has a uniform structure with diamondparticles finer than one micron.

Other objects and features of the present invention will be apparentfrom the following description of the invention with reference to theaccompanying drawings, in which:

FIG. 1 is a microphotograph of a cutting edge formed from a marketedultrafine WC base cemented carbide;

FIG. 2 is a microphotograph of a cutting edge formed from a sintereddiamond compact having a particle size from 3 to 10 micron;

FIG. 3 is a microphotograph showing a structure of a sintered diamondcompact of FIG. 2, a metal binder mainly composed of cobalt lookingwhite, while diamond crystals looking grey;

FIG. 4 is a microphotograph showing a structure of a marketed diamondcompact which is used as a wire drawing die, diamond crystals lookinggrey in contact to one another, while a binder metal mainly composed ofcobalt looking white;

FIG. 5 is a microphotograph showing a surface condition of a copper wirewith a diameter of 0.5 mm which is produced by a die formed from asingle crystal natural diamond;

FIG. 6 is a microphotograph showing a surface condition of a copper wirewhich is produced by a die made from a marketed sintered diamondcompact;

FIG. 7 is a microphotograph showing the inner face of a die of FIG. 6after using, the grey particles being diamond, a wide black area being atorn-off portion of diamond particles;

FIG. 8 is a microphotograph showing a structure of a diamond compactwhich is sintered from a fine diamond powder by a conventional method,the structure presenting numerous diamond crystals which abnormally growto several hundred microns;

FIG. 9 is a microphotograph showing a cutting edge which is formed bygrinding ultrafine sintered diamond compact of the present invention inthe same manner as that of FIG. 2;

FIG. 10 is a microphotograph showing a structure of a sintered compactof the present invention, fine grey particles being diamond, whitebinder portion consisting of WC particles less than one micron;

FIG. 11 is a chart showing grindability in terms of a relative timerequired for grinding a predetermined amount, in which solid circles aresintered compact of the present invention which is produced from adiamond powder finer than one micron bonded by WC-Co cemented carbide,and open circles are sintered compacts for comparison which are producedfrom a diamond powder of average particle size of 6 micron bonded byWC-Co cemented carbide, and an open triangle is a marketed diamondcompact with a binder of cobalt; and

FIG. 12 is a chart showing a tool life in terms of development of flankwear in relation to a cutting time upon cutting Al-13% Si alloy by usinga cutting tool of FIG. 11.

The inventors have made further studies as to the method of producing asintered compact comprising finer diamond particles. As a result, it wasfound that the sintered compact which meets the objects of the presentinvention can be obtained by using a cemented carbide mainly composed ofWC or WC with a slight amount of iron group element, as a binder.

The reason why WC is selected as a binder is as follows: It is of courseimportant to utilize the properties of diamond such as very highhardness, rigidity, wear resistance and highest heat conductivity amongall materials, for applying a sintered diamond compact to a cuttingtool. However, there are a number of difficult technical factors so asto obtain a sintered compact comprising pure diamond since the pressureand temperature required for the sintering is tremendously high. Forthis reason, the diamond powder is sintered by using a suitable binderunder super-pressures, but it is necessary to select a binder which willnot insure the excellent properties of diamond.

WC has the highest rigidity next to diamond or cubic boron nitride, andhas a high heat conductivity. As for the heat expansion coefficientwhich is an another important factor in producing a complex compactcontaining diamond, WC has almost the same value as diamond. Further, inview of the fact that the internal residual stress will not remain inthe sintered compact, WC is an excellent material.

Still further, as to the wear resistance, WC is of course superior tometal binders such as cobalt, but in respect to heat resistance and wearresistance at high temperature, the material may be inferior to someother materials. However, since diamond is unstable at high temperature,the heat resistance of a binder is not so important in this respect.

In consideration of the above points, WC is especially suitable for abinder of a sintered compact of diamond.

Other material having the properties similar to WC is (MoW)C which isobtained by replacing a part or almost all of W in WC by Mo, and whichhas the same crystal structure as WC . One of the inventors has madedetailed studies on the properties of a cemented carbide using thiscompound, and it was confirmed that the properties such as hardness,rigidity, wear resistance, heat conductivity and heat expansioncoefficient of carbides formulated, for example, as (Mc₇ W₃)C or (Mo₅W₅)C are almost similar to WC. Further, the cemented carbides mainlycomposed of these complex carbides can be more plastically deformablebefore fracture, and have higher toughness as well as lower frictioncoefficient all of which are important properties when used as a cuttingtool, as compared to WC-Co cemented carbide.

Although all the description hereinafter is referred to only WC, theabove (MoW)C carbides can also be used quite similarly to WC in thepresent invention.

The tool material according to the present invention is a sinteredcompact which has a uniform structure consisting of diamond particlesfiner than one micron bonded by a carbide finer than one micron mainlycomposed of WC. Since diamond particles as a hard component finely anduniformly disperse in the compact, when the material is formed into acutting tool by grinding, a cutting edge thereof can be very sharpwithout any roughness. An example of the cutting edge is shown in FIG.9. The sintered compact comprises 60 volume % of diamond and the balanceWC finer than one micron. FIG. 10 is a microphotograph showing astructure of this. In order to obtain a sintered compact which hassharpness at a cutting edge as appears from the photographs, WC bindershould be also finer than one micron in particle size.

When the sintered compact of the present invention is used as a cuttingtool, it can be applied to a far wider field than a conventional compactsince the compact of the present invention has sharpness at a cuttingedge, provides a smooth machined surface, has a smaller chipping at acutting edge due to the tough WC binder, and has a smaller cutting forcebecause of the sharper cutting edge which makes possible the use underthe same conditions as cemented carbide cutting tools.

The amount of diamond contained in the sintered compact of the presentinvention ranges from 95 to 20 volume %, and can be varied within therange according to the uses thereof. In a cutting tool of interruptedcutting which particularly requires toughness, a binder-rich compactshould be selected at some sacrifices of wear resistance.

When diamond content is less than 20 volume %, there is no advantage inrespect to cost as well as tool life of a cutting tool which is madefrom a sintered compact produced by a super-pressure apparatus.

Most preferable amount of diamond contained in the compact ranges from70 to 30 volume %. As explained in the following Example 6, the diamondparticles are not in contact to one another in his range. Therefore, thegrindability is considerably improved. Moreover, when the cutting toolof the sintered compact is used for cutting relatively soft Al alloy orCu alloy, the wear resistance thereof will be little reduced, exhibitingexcellent properties.

When the sintered compact of the present invention is used as a wiredrawing die, since the surface of the die is finished highly smoothly,it provides a smaller drawing force which makes it possible to draw afiner and weaker metal wire without difficulty. Further, the surface ofa wire can be highly smooth since there occurs little destroy ortearing-off of the diamond particles of the die.

Meanwhile, the reason why a sintered compact which has a uniformstructure consisting of ultrafine diamond particles less than one microncan be obtained by the method of the present invention is as follows: Adiamond particle has a very high hardness and is difficult to deform.Therefore, when it is compressed under super-pressures, there remainclearances between diamond particles. The finer the diamond particles,the more the clearances. In any sintered compact produced according tothe beforementioned Japanese Patent Publication No. 39-20483 or52-12126, the finer is the diamond powder material, the more bindermetal should be added, or the more binder permeates into the sinteredcompact as a result of sintering. The sintering of a diamond compactaccording to these methods is performed through the liquid phase of aneutectic mixture of carbon of diamond with iron group element, andproceeds with dissolution of diamond into and precipitation thereof fromthe eutectic mixture. Especially, fine diamond particles have a largesurface energy which causes crystal growth, similarly to an ordinaryliquid phase sintering. When abnormal crystal growth occurs uponsintering diamond, there exists about diamond particles a liquid phasesufficient to dissolve them, but no other substances which prevent thecrystal growth.

According to the present invention, however, clearances between diamondparticles finer than one micron are filled with finer WC particles, andby sintering the mix under super-pressures, it is possible to obtain acompletely dense compact without necessity of liquid phase. Since thereexists little liquid phase which is essential for the crystal growth ofdiamond, and since WC particles fill between diamond particles, thecrystal growth is completely depressed during sintering the diamond.

If desired, as a binder for diamond particles can be used WC cementedcarbide which contains, together with WC, a slight amount of iron groupelement. In this case, a slight amount of liquid phase which contains aniron group element in the cemented carbide makes it possible to obtain asufficiently dense compact. WC particles in the cemented carbide preventthe complete adhesion between diamond particles to depress the crystalgrowth. On the other hand, since the adhesion between diamond and WC isstrong, there can be obtained ultrafine sintered compact with a toughbinder of cemented carbide.

In practice, in order to produce the compact of the invention comprisingdiamond and WC crystals finer than one micron, it is convenient to mixdiamond and WC powder by wet ball-milling by using cemented carbideballs in a pot having a cemented carbide lining. Further, an attritor orvibration milling can be employed in the same manner as ball-milling.The mixing operation may be performed for 30 minutes to one week so asto pulverize the material finely.

Since diamond is very hard, a relatively large amount of abraded powderfrom the balls and pot lining mixes in the diamond powder. Therefore, itis convenient to use this as a binder component, especially if thecomposition of the balls and lining are the same as that of a binder.

In the compact of the present invention, a liquid phase produced duringsintering should be minimized. Further, since the binder should have arigidity, it is not preferable to add too much amount of metal into abinder. If the metal is cobalt, 15 weight % is maximum. As a metal otherthan cobalt, iron, nickel or an alloy of cobalt, nickel or iron may beused.

The main component of binder should be WC, but other carbides such asTiC, ZrC, HfC, TaC and NbC can be substituted for a part of WC in therange less than 50 volume %.

A marketed natural or artificial diamond powder for lapping can be usedas a material for the sintered compact of the present invention. Thematerial with a particle size coarser than one micron can be pulverizedby using cemented carbide balls and pot as mentioned before.

When it is necessary to uniformly mix a diamond and WC into a mixturefiner than one micron, the ball-milling is most preferable as describedbefore. In this case, however, since cemented carbide balls and pot areused, a slight amount of binder metal contained in the cemented carbideinevitably mixes. In order to depress the crystal growth of diamondduring sintering, it is preferable to minimize the amount of such ametal. When a large amount of metal is mixed, the powder can be treatedin a hydrochloric acid solution to dissolve and remove the metal.

Hot-pressing should be performed under the conditions of temperature andpressure within the stable range of diamond. This range is well-known asBerman-Simon equilibrium line. Generally, the sintering is performed at1200° to 1600° C. under 40 to 80 Kb for 5 to 60 minutes.

Meanwhile, upon sintering the diamond compact of the present invention,it is necessary to depress the crystal growth of diamond in every way.According to an experiment, when there exists in a binder a slightamount of iron group element such as Co, Fe or Ni together with WC,crystals of diamond and WC tend to grow if the temperature is too high.However, the conditions for producing the sintered compact consisting ofdiamond and WC particles finer than one micron are at temperatures overthe liquid phase generating point of eutectic mixture comprising irongroup element, diamond and WC, but within the range of 100° C. over thepoint. If the metal content in the binder is previously removed by acidas mentioned before, higher temperatures may be applied.

Since the materials of the sintered compact are very fine, a largeamount of gas adsorbs thereon. Therefore, it is necessary to degas byheating the materials in vacuum at temperatures higher than 300° C. Whenthe temperature is lower than 300° C., the treatment requires a longertime, and is not industrially applicable.

The powdered mix for producing the sintered compact of the invention canbe cold-pressed into a green compact before sintering, at a roomtemperature under the pressure of 200 Kg/cm² to 2 t/cm².

In order to illustrate the present invention, references are now to bemade to the following Examples. Throughout the Examples, percentages arethe percentages by weight unless otherwise specified.

EXAMPLE 1

Ultrafine diamond powder having a particle size finer than one micronwas pulverized in a solvent of acetone by using WC-7% Co cementedcarbide balls and a pot having the same cemented carbide lining. Theamount of diamond was 5 g, but after 40 hours of ball-milling, thepowder weighed 8.3 g. This increase was caused by the fine cementedcarbide powder abraded from the balls and pot.

The diamond amount contained in the powder mix was estimated to be 80volume %. As a result of examination by a scanning electron microscope,it was confirmed that all the particles in the mix were finer than onemicron.

The powder mix was cold-pressed into a disk having a thickness of 1.5 mmand diameter of 10 mm. The disk was treated in a vacuum furnace at 1000°C. to degas, and thereafter was sintered under 55 Kb at 1370° C. for 10minutes by means of a super-pressure apparatus for making a diamond.

The sintered compact was lapped by a diamond paste, and the structurethereof was examined to be found that the compact comprises ultrafinediamond and WC particles less than one micron in size.

The compact was cut. A piece thereof was brazed to a steel shank, and acutting edge was ground by a diamond wheel. For comparison, a cuttingtool of the same shape was made from a marketed sintered diamondcompact.

The ground cutting edges were observed. The cutting edges of themarketed compact had a number of chippings with a width of about 10micron, which might be caused during grinding.

A copper alloy commutator of an electric motor was cut by both of thecutting tools at a cutting speed of 400 m/min, depth of cut of 0.5 mmand a feed of 0.05 mm/rev. After cutting 2000 pieces by the cutting toolof the invention, the surface roughness of a workpiece was 1.3 micron atmaximum, while a workpiece cut by the cutting tool of marketed compacthad initial surface roughness of 2.6 micron at maximum, and aftercutting 500 pieces, the surface roughness reached 3.9 micron.

EXAMPLE 2

A diamond powder material same as Example 1 was pulverized for 120 hoursby using the same cemented carbide balls and pot as Example 1. A diamondpowder of 5 g became 19.2 g by the increase of 14.2 g. The compositionof powder was estimated to be 60 volume % of diamond and the balanceWC-7% Co. A metal component was dissolved and removed from the powdermix by a diluted hydrochloric acid.

After cold-pressing the powder mix, it was subjected to degassing in thesame manner as Example 1.

Further, there were prepared a WC-10% Co cemented carbide disk having athickness of 3 mm and diameter of 10 mm together with a molybdenum diskhaving a thickness of 0.05 mm and diameter of 10 mm. The green compactcontaining diamond was put on the molybdenum disk under which wasdisposed the cemented carbide disk. The assembly was inserted in asuper-pressure apparatus, and sintered in the same manner as Example 1.

The sintered compact was cut, and the sectioned face was examined to befound that a sintered compact with a thickness of 1 mm containingultrafine diamond particles firmly engages the cemented carbide diskthrough the intermediary of a 50 micron thick layer consisting ofmolybdenum carbide. The structure of the sintered diamond part is shownin a microphotograph of FIG. 10.

Cutting tools were prepared from the sintered compact of the inventionand a marketed sintered compact of which the particle size of diamond is3 to 10 micron. A round bar of Al-18% Si alloy having longitudinal slitswas cut by the cutting tools at a cutting speed of 500 m/min, depth ofcut of 0.13 mm and a feed of 0.05 mm/rev. After 30 minute cutting by thetool of the invention, the flank wear width was 0.15 mm which wasnormal, while the cutting tool made from the marketed compact had a 0.5mm wide chipping at the flank after 30 minute cutting.

The machined surface of a workpiece cut by the tool of the invention hada roughness of 1.6 micron at maximum in the initial stage of cutting,while the surface cut by the marketed compact was 2.6 micron at maximum.

EXAMPLE 3

There were prepared two kinds of powders by pulverizing a diamond powderwith particle size of 3 to 6 micron for 5 hours and for 25 hoursrespectively, in the same manner as Example 1. The former contained32.1% of WC-7% Co cemented carbide and about 90 volume % of diamond. Thelatter contained 86.5% of cemented carbide and 40 volume % of diamond.

Both of the powder mix were sintered in the same manner as Example 1 toobtain sintered compacts having a diameter of 3 mm and thickness of 1.5mm. As a result of examination, it was confirmed that both compactsconsist of diamond and WC particles finer than one micron. The compactswere formed into dies having a hole of 0.5 mm of diameter. A similar diewas prepared from a marketed compact having coarser diamond particles ofabout 60 micron.

Aluminium wire was used as a material to be drawn, and the drawing forceof each die was measured using a spindle oil as a lubricant. The die ofmarketed compact exhibited 15.1 Kg/mm² of drawing force, the diecontaining 90 volume % of diamond of the invention being 12.1 Kg/mm²smaller than the former, the other die of the invention being 13.8Kg/mm². Further, the surface of drawn wires were compared to oneanother. The wires drawn by the dies of the invention had less scratchesthan those drawn by the die of marketed compact, especially the wiresdrawn by the die of the invention containing larger amount of diamondhad fewer surface scratches.

EXAMPLE 4

4 g of diamond powder of Example 1 added with 1 g of TaC powder having aparticle size of 3 micron was pulverized for 120 hours by using ballsand pot consisting of (Mo₇ W₃)C-10% Co-5% Ni alloy. After theball-milling, the powder weighed 15 g. A metal component mixed wasremoved in the same manner as Example 2. The powder mix comprised 65% ofdiamond, 32% of (Mo₇ W₃)C and 3% of TaC, all of the volume base.

The powder mix was sintered in the same manner as Example 2 under 55 Kbat 1450° C. for 10 minutes to obtain a sintered compact bonded to acemented carbide disk through the intermediary of molybdenum carbidelayer. The obtained compact was a ultrafine alloy having a uniformstructure consisting of fine diamond particles less than one micron,(Mo₇ W₃)C finer than one micron and a slight amount of TaC.

EXAMPLE 5

5 g of diamond powder having a particle size of 2 to 6 micron waspulverized for 72 hours by using WC-8.1% Co cemented carbide balls andpot. The obtained powder weighed 36.8 g by the increase of 31.8 g ofcemented carbide fine powder abraded from the balls and pot duringball-milling.

The analysis of the powder mix revealed that it comprises 40 volume % ofdiamond, 51.7 volume % of WC and 8.3 volume % of Co. After heat-treatingthe mix so as to degas in the same manner as Example 1, it was sinteredunder 55 Kb at 1400° C. for 10 minutes by a super-pressure apparatus inthe same manner as Example 2. The obtained compact was examined by amicroscope to be found that diamond particles finer than one micronuniformly disperse in the structure and WC particles in the binder ofWC-Co alloy are finer than 0.5 micron. Vickers hardness of the compactwas 3300 Kg/mm².

The compact was cut and brazed to a steel shank to form a cutting tool.For comparison, cutting tools of the same shape were prepared by using asingle crystal natural diamond and a marketed sintered compact havingdiamond particles of 3 to 10 micron, respectively. A piston of engineconsisting of Al-Si alloy was cut at a cutting speed of 250 m/min, depthof cut of 0.2 mm and feed of 0.15 mm/rev. The cutting tool formed fromthe compact of the invention exhibited the performance equivalent tothat formed from natural diamond in both the surface roughness anddimension precision. Further, 3600 pieces were cut by the cutting toolof the invention before the tool life expires, while 1800 pieces by thecutting tool made from the natural diamond due to chipping at thecutting edge. On the other hand, the cutting edge made from the marketedsintered compact did not exhibit sharpness at the initial stage ofcutting, and could not be applied to actual use since the machinedsurface was too rough.

EXAMPLE 6

In order to examine the grindability characterizing the compact of theinvention as well as the relation between the tool life and composition,various compacts were prepared by varying particle size of diamondpowder and amount thereof.

There were prepared two kinds of sintered compacts, one having aparticle size of about 6 micron and the other finer than one micron,each of compacts containing different amount of diamond, by adjusting aparticle size of diamond powder material, amount thereof before puttingin a ball-milling pot, number of cemented carbide balls, and a time forball-milling. As a binder, solely WC-8% Co alloy was used. The sinteringconditions were the same as Example 5.

The grindability was examined with respect to each of the sinteredcompacts with a diameter of 10 mm different in particle size and diamondcontent.

The time required for grinding the compact by 0.1 mm in the direction ofthickness was measured by using a diamond wheel (270/300 mesh of diamondgrain size) attached to a surface grinding machine at a wheel surfacespeed of 1700 m/min and grinding depth of 0.002 mm/path. FIG. 11 showsthe results. In the chart, open triangle is the marketed diamondcompact, solid circles being the compacts containing diamond particlesfiner than one micron, open circles being the compact for comparisoncontaining diamond particles of about 6 micron. The times are relativetime when the time required for grinding the marketed compact by 0.1 mmwas regarded as 100. As seen, the grindability of the compacts of thepresent invention were remarkably improved.

FIG. 12 shows the results of cutting time required for reaching apredetermined wear width by using the cutting tools formed from the samecompacts as FIG. 11 upon cutting Al-13% Si alloy. As apparent, thesmaller the amount of diamond in the compact, the shorter the tool life.However, as is noted in consideration of FIG. 11, the sintered compactof the invention containing diamond particles finer than one micronexhibited better grindability as well as better wear resistance whenused as a cutting tool, as compared to the sintered compacts of coarserdiamond particles.

When the sintered compact of the invention contains less than 70 volume% of diamond, the grindability is remarkably improved, as shown in FIG.11. The reason is that the diamond particles in the structure of thecompact are not contact with each other.

What we claim is:
 1. A method of producing a sintered compact for use ina cutting tool comprising:pulverizing a diamond powder into a particlesize finer than one micron by using cemented carbide balls and a pothaving a cemented carbide lining, the cemented carbide being selectedfrom the group of WC base cemented carbide and (MoW)C base cementedcarbide; mixing the diamond powder with a powder abraded from the ballsand pot, which is also pulverized into a particle size finer than onemicron to produce a powder mix containing 95 to 20 volume % of thediamond powder; degassing the powder mix in a vacuum at temperaturesover 300° C; and hot-pressing the powder mix under high pressure at hightemperature within the stable range of the diamond powder.
 2. A methodof producing a sintered compact as claimed in claim 1, furthercomprising adding a cemented carbide powder selected from the group ofWC base cemented carbide powder and (MoW)C base cemented carbide powderinto the diamond powder before pulverizing.
 3. A method of producing asintered compact as claimed in claim 1, further comprising adding acarbide powder selected from the group of WC and (MoW)C powder into thediamond powder before pulverizing.
 4. A method producing a sinteredcompact as claimed in claim 1, further comprising cold-pressing thepowder mix before degassing.
 5. A method of producing a sintered compactas claimed in claim 1, wherein the powder mix contains 70 to 30 volume %of the diamond powder.